top scientists at the united nations say they want you to look at the pictures they have been studying, planet earth, the air, the water, the soil. Hundreds of scientists saying tonight, it is visual proof of global warming, and what we all have to do next. Abc's dan harris with today's big announcement. Reporter: Devastating floods in colorado, heat records in las vegas. Superstorm sandy, melting glaciers, dying coral reefs. We will see much more of these kinds, which they pin with near certainty on us humans. It is extremely likely, for the past half century are due to human influence. Reporter: Rising heat already costs the average american family like the rieblers of new jersey in everything from food to air-conditioning. But that is nothing says today's report if the world doesn't make big changes soon by the year 2040, we could have released enough greenhouse gases, that we will have locked in a global temporize of 3.6 degrees. Which is where potentially irreversible and catastrophic changes kick in. We're talking about an increase in animal extinction, a drying up of farm land that leads to massive food shortages, and putting us on a path toward sea level rise, that could swallow parts of major american cities like new york, miami and san diego. Skeptics have predictively accused the u.N. Panel of being alarmist. But this is a major wake up call. Why isn't there a greater sense of urgency out here in the real world about climate change? A concern that is mostly in the future is something you can easily put off and not worry about today. But this is bearing down on us. It's coming at us like a mack truck. Reporter: So what kind of changes need to be made? How will they affect you? Experts say we need to shift to cleaner energy like natural gas, wind and solar, and move toward more energy efficient cars and home appliances, some of this could be more extensive for us, frankly. But it is definitely cheaper than run away global warming. They were unequivocal today. Unequivocal. Thank you, dan.

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